A method for the study of light-responses (phototactic behavior) in third instar larvae of Drosophila is described. Preliminary experiments show that larvae from populations of the same and from different species vary in their response to light and there is a genetic component for this variation. Virtually nothing is known about the genetic and environmental factors influencing the photoactic behavior of Drosophila larvae. Surprisingly, little information is available which deals specifically with the structure and function of Drosophila larval photoreceptors. Experiments are proposed to 1) determine the structure of the Drosophila larval ocelli at the level of the light microscope and electron microscope, 2) determine how much variation in larval phototaxis exists in natural populations of various Drosophila species, 3) to look at possible correlations between larval and adult light responses, 4) to do a genetic analysis of larval phototactic behavior and 5) to examine the influences of various environmental factors such as light intensity and wavelength, temperature, humidity and the presence of larvae from other species on the phototactic behavior of third instar larvae.